The present invention concerns a procedure for controlling cellulose digestion by measuring the activity of chemicals which essentially affect the cellulose digestion and which are present in a cellulose digester, with supplying of ingredients or chemicals to be added into the cellulose digester being controlled on the basis of the measurement results. The present invention is also directed to a method for measuring the cellulose digestive activity itself within the cellulose digester.
Information on various processing variables is required in the control of cellulose digestion. The progress of the digesting process is also followed with the aid of dissolving lignin, in addition to control of the incoming and outgoing pulp flow. Determining the lignin content of the pulp itself, would best reflect the progress of digestion. However, such determination is too time-consuming. It is moreover exceedingly difficult to obtain a representative sample of the pulp during the digestion. For these reasons, the monitoring of digestion has been restricted to analyzing the chemicals concentrations of the digesting solution. The variable on which attention is most commonly focussed is the active or effective alkali. The most common analytic method for determining the effective alkali is based on titration. Perhaps best known among computer-controlled analyzers of effective alkali in continuous-action digestion, is the apparatus in which titration is carried out with carbon dioxide. Titration is effected under pressure by measuring the reaction temperature and determining the end-point of titration from a titration graph with the aid of a computer. Apparatus based on calorimetric titration has also been developed for analysis of effective alkali.
A drawback encumbering these methods is that in the sampling, process samples cannot be taken at arbitrary points in the cellulose digester, so that the sample is not sufficiently representative. Moreover, analysis is based on content, not on activity. This detracts from the reliability of results. The apparatus designs are expensive and complex, this meaning high chances of error. Maintenance also involves considerable expense.
Conductivity measurement has been increasingly employed towards measuring effective alkali, this method being simple and inexpensive. However, several factors exert influence on conductivity, such as temperature, sodium carbonate, and sodium sulfide, so that changes in the same have to be taken into account.
However, the selectivity of conductivity measurement is not good enough, because the measuring technique fails to react sufficiently swiftly to ions. Problems and errors in measurement are furthermore caused by electrode contamination. Cleaning and recalibration and measurement which are necessary thereafter, increase the operating costs.
The use of an ion-selective electrode is based on the activity of the ion, which can be thought of as a kind of effective concentration. The potential which the electrode yields is a function of the logarithm of activity. Best known among ion-selective electrodes is the pH pick-up, its output value being a function of the logarithm of the hydrogen ion activity.
The advantages of a measuring method based on ion selectivity include the placeability of pick-ups in the process flow, whereby there is no need to provide for sampling, which is often quite cumbersome. With ion-selective electrodes used in conjunction with the sulfate process, slightly better selectivity of information is achieved as compared with other analytic methods which are practiced, although this selectivity is still not good enough. For this application, sodium and sulfide electrodes are commercially available, in addition to the pH electrode.
All the methods outlined in the foregoing have either the drawback that they require inconvenient sampling, or the drawback that the results of analysis are not selective enough.